Wireless access systems have been widely deployed in order to provide various types of communication services including voice or data. In general, a wireless access system is a multiple access system that can support communication among multiple users by allowing them to share available system resources (a bandwidth, transmission power, etc.). Examples of multiple access systems include Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), Multi Carrier Frequency Division Multiple Access (MC-FDMA), etc.